The PDM system has been designed to capture, measure and log the ultra high frequency (UHF) signals generated by partial discharges within gas insulated switchgear (GIS). Couplers are distributed throughout the switchgear to pick up signals in the range of between 500 MHz to 1,500 MHz. These signals are routed to the PDM where the signal is detected, amplified and stored for diagnosis. From the stored data it is possible to determine the magnitude, probable location and type of any partial discharge activity within the GIS.